This is totally random but I've had what my Chiropractor thought was a sacroiliac issue. Symptoms very much like yours. About two months ago I was diagnosed with an inguinal hernia. Had the surgery last Thursday. That morning my back was killing me. After the surgery I noticed that I no longer had the back pain. It has not come back. I did a search and it is possible the back pain was caused by the hernia. My Chiropractor confirmed. Just something to consider.